The production of regular grain oriented electrical steel requires critical control of all the processing steps to provide material having the desired magnetic properties which are stable and reproducible. The present invention has found a combination of processing steps which produce (110)[001] oriented electrical steel using a single stage of cold reduction while providing magnetic quality previously obtainable only with a two stage cold reduction process.
Grain oriented electrical steels are characterized by the level of magnetic properties developed, the grain growth inhibitors used and the processing steps which provide these properties. Regular or conventional grain oriented electrical steels typically have magnetic permeability below 1880 as measured at 796 A/m. High permeability grain oriented electrical steels have magnetic permeability of 1880 or above and as such are differentiated from regular grain oriented electrical steels. As taught in the prior art, regular grain oriented electrical steels are produced using manganese and sulfur (and/or selenium) as the principle grain growth inhibitor(s) with two cold reduction steps separated by an annealing step. Aluminum, antimony, boron, copper, nitrogen and other elements are sometimes present and may supplement the manganese sulfide/selenide inhibitor(s) in amounts insufficient to provide the needed level of grain growth inhibition.
Representative processes for producing regular grain oriented electrical steel are taught in U.S. Pat. Nos. 3,764,406; 3,843,422; 4,202,711 and 5,061,326 which are incorporated herein by reference. Most regular grain oriented electrical steel strip or sheet is produced using a two stage cold reduction process because it typically provides better and more uniform magnetic properties. While a single stage cold reduction process has long been sought since it eliminates at least two processing steps, the magnetic properties have not been obtainable with the same degree of consistency and quality.
Regular grain oriented electrical steel may have a mill glass film, commonly called forsterite, or an insulative coating, commonly called a secondary coating, applied over or in place of the mill glass film, or may have a secondary coating designed for punching operations where laminations free of mill glass coating are desired in order to avoid excessive die wear. Generally, magnesium oxide is applied onto the surface of the steel prior to the high temperature anneal. This primarily serves as an annealing separator coating; however, these coatings may also influence the development and stability of secondary grain growth during the final high temperature anneal and react to form the forsterite (or mill glass) coating on the steel and effect desulfurization of the base metal during annealing.
To obtain material having a high degree of cube-on-edge orientation, the material must have a structure of recrystallized grains with the desired orientation prior to the high temperature portion of the final anneal and must have grain growth inhibition to restrain primary grain growth in the final anneal until secondary grain growth occurs. Of great importance in the development of the magnetic properties of electrical steel is the vigor and completeness of secondary grain growth. This depends on having a fine dispersion of manganese sulfide or other inhibitor which is capable of restraining primary grain growth in the temperature range of 535.degree.-925.degree. C. (1000.degree.-1700.degree. F.). Thereafter, the cube-on-edge nuclei have sufficient energy to develop into large secondary crystals which grow at the expense of the less perfectly oriented matrix of primary grains. The dispersion of manganese sulfide is typically provided by high temperature slab or ingot reheating prior to hot rolling during which the fine manganese sulfide is precipitated.
The production of cube-on-edge oriented electrical steel requires that the material be heated to a temperature which dissolves the inhibitor prior to hot rolling so that during hot rolling the inhibitor is precipitated as small, uniform particles. U.S. Pat. No. 2,599,340 disclosed the basic process for the production of material from ingots and U.S. Pat. Nos. 3,764,406 and 4,718,951 obtained good magnetic properties from material which was continuously cast as slab followed by heating and hot rolling the cast slab prior to the conventional hot rolling step to reduce the size of the columnar grain structure.
Work done in the past, as represented in U.S. Pat. No. 3,333,992 (incorporated herein by reference), added large amounts of sulfur during the early portion of the final high temperature anneal by providing a sulfur-bearing annealing atmosphere or surface coating or both. However, achieving permeabilities at 796 A/m consistently in excess of 1800 required at least two cold reduction stages separated by an annealing step. In the examples of U.S. Pat. No. 3,333,992, a high level of manganese in excess of that required to combine with sulfur and/or selenium from the melt stage was employed.
U.S. Pat. No. 4,493,739 teaches a method for producing regular grain oriented electrical steel using one or two stages of cold rolling. This patent teaches the use of 0.02-0.2% copper in combination with control of the hot mill finishing temperature to improve the uniformity of the magnetic properties. Phosphorus was controlled to less than 0.01% to reduce inclusions. Tin up to 0.10% could be employed to improve core loss of the finished grain oriented electrical steel by reducing the size the (110)[001] grains. The manganese sulfide precipitates were considered to be weak and the uniformity of the magnetic properties were improved by forming fine copper sulfide precipitates to supplement the manganese sulfide inhibitor. During hot rolling, the finish hot strip rolling entrance and exit temperatures were controlled to be from 1000.degree.-1250.degree. C. and 900.degree.-1150.degree. C., respectively. The examples of U.S. Pat. No. 4,493,739 show a conventional two stage cold rolling process was used. While the manganese and copper sulfide precipitates formed after hot rolling were fine and uniformly dispersed, the heavy 60-80% cold reductions required for grain size control and texture development in U.S. Pat. No. 4,493,739 implied that unstable secondary recrystallization would result with a single stage of cold reduction process although no such examples are shown.
U.S. Pat. No. 3,986,902 is related to excess manganese in regular grain oriented electrical steel. The patent uses manganese sulfide for the grain growth inhibitor needed for secondary recrystallization. To be effective, these inhibitors must be finely dispersed to prevent grain boundary migration and grain growth during primary recrystallization and promote grain growth of the (110)[001] grains during secondary recrystallization. Hot working causes these precipitates to grow appreciably and to be concentrated intergranularly such that the precipitates are less effective as grain growth inhibitors. It is therefore essential that the precipitates be dissolved in solid solution and that they precipitate as finely dispersed particles during or after the final step of hot rolling to band. Prior art practices discussed in this patent reviewed the need to provide a silicon steel with 0.07-0.11% manganese and 0.02-0.4% sulfur to provide the necessary grain growth inhibitors (0.055-0.11% manganese sulfide). Manganese in excess of that required to combine with sulfur to form manganese sulfide was present. The excess manganese was desired to prevent hot shortness; however, the patent taught that higher excess manganese decreased the solubility product of manganese sulfide and required higher slab or ingot reheating temperatures since the manganese sulfide was more difficult to dissolve. The patent sought to lower reheating temperatures to 1250.degree. C. (2290.degree. F.) or less by reducing the solubility product to a maximum of about 0.0012%. To enable effective grain growth inhibition using a smaller amount of manganese sulfide further required lowering the levels of insoluble oxides, such as Al.sub.2 O.sub.3, MnO, FeSiO.sub.3, etc., in the steel. It was believed that the oxides had very low solubility in solid steel, particularly at the lower reheating temperatures desired by this invention. Sulfur also had a tendency to react with the oxide inclusions and form oxysulfides, negatively influencing the solubility limits and affecting the development of the desired cube-on-edge orientation. The oxide inclusions noted in U.S. Pat. No. 3,986,902 were incurred during melting and teeming.
Various prior art attempts have been made to reduce the oxygen content to minimize such inclusions such as U.S. Pat. No. 3,802,937 which used lower amounts of manganese sulfide while minimizing oxide nucleation, particularly through the use of protection of the pouring stream during the teeming to avoid reoxidation products. The patent required that the manganese sulfide solubility product be maintained at less than 0.0012% and preferably from 0.0007-0.0010%. This was accomplished, for example, by using 0.05% manganese and 0.02% sulfur. Reducing either sulfur, manganese or both served to provide a lower solubility product; however, since the sulfur must be removed in the final anneal, it was preferred to keep sulfur low and maintain a controlled level of manganese. This resulted in a process having about 0.07-0.08% manganese and about 0.011-0.015% sulfur, the excess manganese content insuring that all of the sulfur was combined as manganese sulfide. As previously mentioned, control of the reoxidation products enabled using lower levels of manganese and sulfur with the lower slab reheating temperatures. Lower manganese-to-sulfur ratios of about 1.7 could be used while avoiding hot brittleness as compared with previous practices in the art which required ratios of about 3.0 . Per the teachings of U.S. Pat. No. 3,802,937, the slabs were reheated to a temperature of less than 1260.degree. C. (2300.degree. F.) and hot rolled to 1.3-2.5 mm (0.05-0.10 inch) thickness before the temperature falls to between 790.degree.-950.degree. C. (1450.degree.-1750.degree. F.). After hot rolling, the steel is cooled to between 450.degree.-560.degree. C. (850.degree.-1050.degree. F.) prior to coiling. Annealing of the hot rolled bands at a temperature of at least 980.degree. C. (1800.degree. F.) was preferred but optional. The bands were cold reduced to an intermediate thickness, annealed and again cold reduced to a typical final thickness of about 0.28 mm (0.011 inch). The steel was then decarburized at a temperature of 760.degree.-815.degree. C. (1400.degree.-1500.degree. F.) to reduce the carbon to 0.007% or less and provide primary recrystallization and subjected to a final anneal at about 1065.degree.-1175.degree. C. (1950.degree.-2150.degree. F.) to effect secondary recrystallization. The one example used 0.031% carbon, 0.055% manganese, 0.006% phosphorus, 0.02% sulfur, 2.97% silicon, 0.002% aluminum, 0.005% nitrogen and balance iron.
As pointed out by the above patents, the control of the manganese sulfide precipitates and the various processing steps required for producing regular grain oriented electrical steel having uniform and consistent magnetic properties is difficult. The ability to obtain the desired properties using a single cold reduction process is even more difficult and it is this challenge to which the present invention is directed.